All hydrogen-containing gas mixtures that can be used as starting materials of a synthesis reaction are in principle termed synthesis gas. Typical syntheses for which the synthesis gas is used are the syntheses of methanol and ammonia. The breakdown of the synthesis gas into the individual components CO, H2, CO2, H2O and CH4 likewise forms a broad field of application for reforming plants.
The production of synthesis gas can in principle proceed from solid, liquid and gaseous starting materials. The most important method for generating synthesis gas from gaseous reagents, what is termed reforming, generally utilizes natural gas as reagent. Natural gas is substantially a mixture of gaseous hydrocarbons, the composition of which varies depending on the site of origin, where the main component is always methane (CH4) and, as further components, higher hydrocarbons having two or more carbon atoms and also impurities such as sulphur, for example, can be present.
To reform natural gas to form a synthesis gas, what is termed steam reforming is principally used, in which the methane that is present is converted in the presence of a catalyst primarily according to the following reaction equations to form hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2):CH43H2OCO+3H2 andCO+H2OCO2+H2 
When a suitable catalyst is used, and steam is added, moreover, cleavage of higher hydrocarbons to form methane occurs according to what is termed the rich gas reaction:
                    C        n            ⁢              H        m              +                  (                              2            -            n                    2                )            ⁢              H        2            ⁢      O        ↔                    (                  n          -                                                    2                ⁢                n                            -              m                        4                          )            ⁢              CH        4              +                                        2            ⁢            n                    -          m                          4          ⁢                      CO            2                              .      
The highly endothermic character of the methane conversion with water to form carbon monoxide dominates the overall enthalpy of steam reforming. The energy input necessary for this process, which is therefore endothermic overall, is generally realized via an external heater, what is termed the furnace. For this purpose, the overall stream of the carbonaceous energy carrier is divided and the first part is introduced as reagent into the steam reforming, while the second part is fed as fuel gas into the furnace. In principle, also, different carbonaceous energy carriers can be fed into the steam reforming and into the furnace.
The methane conversion can be increased by increasing the steam-carbon ratio, i.e. by superstoichiometric addition of steam.
The subsequent purification with synthesis gas is dependent on the composition of the synthesis gas. If the synthesis gas contains hydrogen which is to be used further in downstream processes, then, to purify the hydrogen, pressure-swing adsorption (frequently also PSA) is usually carried out.
Pressure-swing adsorption (PSA) is a physical method for separating gas mixtures under pressure by means of adsorption. Special porous materials such as, for example, zeolites or activated carbon are used here as adsorbents. The gas is introduced at an elevated pressure of 10 to 80 bar into a fixed-bed reactor, which is filled with the adsorbents, and so the gas flows through the fixed bed. One or more components of the mixture, termed the heavy components, are then adsorbed. At the exit of the bed, what is termed the light component, in this case hydrogen, can be taken off. After some time, the adsorber bed is substantially saturated, and some of the heavy components co-exit. At this time, via valves, the process is switched over in such a manner that the exit for the light component is closed and an outlet for the heavy components is opened. This is accompanied by a pressure fall. Via the low pressure, then, the adsorbed gas is desorbed again and can be taken off at the outlet. Generally, two alternately loaded and discharged adsorbers are connected, and so continuous operation is possible.
If the synthesis gas also contains carbon monoxide, the purification generally additionally comprises a CO2 removal, a synthesis gas drying unit and a low-temperature synthesis gas separation plant or cryogenic synthesis gas separation plant (CO Cold Box).
The CO2 removal is usually a scrubber in which amines or else carbonates are used as scrubbing medium. In this case, in a first step, the CO2 accumulates in the scrubbing medium and can then, together with the scrubbing medium, be transferred to a second separation step. In the second step, scrubbing medium and CO2 are separated again from one another, whereby the CO2 can be taken off in concentrated form.
In the drying, water still originating from the steam reforming is removed from the synthesis gas. The drying unit is usually a temperature-swing adsorption unit (TSA).
In the low-temperature separation plant, the CO gas is separated off from the other components still present in the synthesis gas.
In addition to the CO gas, in the low-temperature separation plant, a hydrogen-rich stream and one or more residual gas streams are generated. For purifying the hydrogen-rich streams, a pressure-swing adsorption is usually used, which separates off the hydrogen from what is termed the PSA residual gas.
In accordance with the possible connection variants of the individual purification stages, the residual gas contains unreacted methane, higher-value hydrocarbons, CO2, H2O, inert gases, such as nitrogen and argon, and also unseparated hydrogen.
The residual gas is recirculated to the furnace of the endothermic steam reforming. The amount of additional fuel gas required can therefore be reduced, since the residual gas usually possesses a not insignificant heating value. Furthermore, in this manner the residual gas need not be worked up further or burnt in a flare.
U.S. Pat. No. 2,667,410 describes a method for controlling the quality of the synthesis gas generated from a steam reforming process, with the focus on the residual gas formed in the purification. In this case, by measuring the unreacted methane and the reaction temperature, the amount of the methane supplied is adjusted in such a manner that the synthesis gas formed always has the same composition, which is necessary for the downstream ammonia synthesis.
US 2010/0255432 A1 discloses a method for starting up a steam reforming reactor, in which a mixture of a fuel gas and an inert gas is generated which has a composition such that the heating output corresponds to 25% of the heating output in the steady-state operation with recirculation of a residual gas. The oven of the steam reformer is operated during startup using this mixture.
A problem with the recirculation of residual gas is that with the cleaning devices, malfunctions can always occur. The steam reforming reactor, however, is very sensitive to pressure fluctuations in the furnace chamber. The control range is usually at a slight under pressure of from −1 to −10 mm of water. In the event of malfunction of one of the gas cleaning processes, the malfunctioning cleaning stage and also possibly all subsequent cleaning stages must be turned off immediately. Usually, then, the crude synthesis gas is simply burnt in a flare. Owing to the loss of the residual gas, when one or more cleaning stages are switched off, the total amount of fuel gas introduced into the furnace falls, this gas being composed of fuel gas and recirculated residual gas during standard operation. This leads to a reduced pressure in the combustion chamber, as a result of which emergency shutdown of the entire reformer plant may occur.
A problem here is that in the event of malfunction of the entire steam reforming, idle times of 24 hours and more occur. This far exceeds the idle times which are necessary for the renewed startup of the cleaning stages (for example low-temperature synthesis gas separation plant 4 to 8 hours, CO2 scrubbing 1-3 hours, pressure-swing adsorption 1 to 3 hours). Therefore, it is absolutely necessary to avoid the entire process being switched off owing to loss of one of the recirculating streams from one of the cleaning plants.
The problem of sudden pressure fall in the event of loss of a residual gas originating from the purification has in practice to date been generally solved in that a false air flap is opened on the steam reformer furnace, through which false air is drawn into the furnace.
A disadvantage with this procedure, however, is that by drawing in of cold ambient air, the temperature in the furnace chamber and flue gas waste-heat system falls greatly. Therefore, considerable temperature fluctuations can occur on the furnace side and process side of the reformer furnace, for which reason the reforming process must then finally be switched off anyway.